AU595151B1 - Microwavable containers useful for controlled heating - Google Patents

Abstract

A container useful in microwave heating of foods comprises a substrate with a titanium nitride film on at least a portion of the substrate, the substrate being substantially microwave transparent except where coated with the titanium nitride film and the film being adapted to absorb microwave energy so that in use the film becomes heated and transfers heat to foods carried by the container, so as to provide crisp heating of the foods.

Description

COMMONWEALTH OF AUSTRALIA PAThNTS ACT 1952 COMPLETE SPECIFICATION 0 P. 5

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9 a, 'I I NAME ADDRESS OF APPLICANT: The BOC Group, Inc.

575 Mountain Avenue Murray Hill, New Providence New Jersey 07974 United States of America NAME(S) OF INVENTOR(S): Don GRIFFIN Steven J. NADEL Tainzen Lee Van SKIKE atkff ADDRESS FOR SERVICE: DAVIES COLLISON Patent Attorneys I Little Collins Street, Melbourne, 3000.

This document contains the amendments made und-r Section 49 and is correct for printing.

COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: Mcrowavable containers useful for controlled heating The following statement is a full performing it known to me/us:description of this invention, including the best method of

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5 Field of the Invention The present invention relates to microwavable containers, and more particularly to microwavable containers with control elements having reflective and absorptive properties to provide crisp heating of foods.

Background of the Invention Non-conductive materials, such as paper, plastic and glass, are substantially microwave transparent as they neither reflect nor absorb microwaves. While containers for some foods in these "transparent" materials are acceptable when the foods are heated, surfaces of foods heated in a microwave oven tend to become soggy due to condensation of water vapor on the food surface. This is a problem for foods that are desirably crisp--such as fried chicken, french S2C fries, pizza and the like. As a consequence, metallized polyester films have been developed as susceptors (thin films of a conductive material designed to absorb microwave energy) in order to heat food adjacent the metallized film area by conduction and radiation. The metallized area, because of its low heat mass, heats quickly and transfers the heat for browning and crisping. Thus, susceptors assist in heating food surfaces to prevent water vapor condensation and soggy food surfaces.

1ai 11 12 13 14 16 17 0* S18 19 20 21 22 23 24 25 S' 26 27 28 29 31 32 33 34 36 37 FA4/.38 2 Thin coatings of aluminum on polyester film are known and used as such susceptors by adhesive lamination to paper or paperboard. However, aluminum oxidizes over time, thus losing its conductivity, and aluminum also continues to absorb microwave energy. This creates the possibility of overheating. Controlling the amount of metal deposited on the polyester is also difficult. The adhesive lamination can pose problems due to outgassing from the adhesive during heating which causes odors and undesirable tastes.

Summary of the Invention It is an object of the present invention to make microwave susceptor films that absorb and reflect on organic substrates, yet which maintain a temperature less than that of the organic substrate during normal microwave heating.

In one aspect of the present invention, a container useful in microwave heating of foods comprises a substrate with a titanium nitride film on at least a portion of the substrate. The substrate is substantially microwave transparent while the film is adapted to absorb microwave energy and thus become heated, and to transfer heat to foods carried by the container. A preferred substrate is crystallized polyethylene terephthalate, which has a melting point .between about 198 0 C-218 0 C. With titanium nitride films of the invention, containers can be heated in conventional microwave ovens (such as having an incident power of about 1600 watts at 2450MHz) for crisp heating of foods, yet without causing tray melting.

Brief Description of the Drawing Fig. 1 is a perspective view of a container embodiment of the invention, given by way of example only.

900110.eldspe.00339924.spe, 3 Detailed Description of the Preferred Emibodiments Containers of the invention comprise a substrate that is substantially microwave transparent and has a determinable melting or scorching temperature.

For example, substrates formed of crystallized polyethylene terephthalate (CPET) have a melting or scorching temperature between about 198 0 C-218 0 C, while paper board and plastic/paper composite substrates typically have a heat resistance up to about 205 0

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Such plastic, paper and composite substrates are suitable for use as substrates of the present invention, although CPET is particularly preferred due to an attractive, table-ready appearance. It should be understood, however, that a wide variety of relatively :0"406 15 heat stable, but organic, substrates are suitable.

Turning to Fig. 1, a container 10, useful in microwave heating of foods, comprises substrate 12 (as has been described) with a titanium nitride film 14 on at least a portion of substrate 12. As illustrated, substrate 12 may be formed as a tray with a first food two receiving cavity 16 and a second food receiving cavity 18. Film 14 may be adhered to the bottom 20 of cavity 16 so that when foods are placed within the cavity 16, then film 14 absorbs microwave energy, becomes heated, and transfers heat to the! foods.

Bottom 20 can be substantially flat, as illustrated, or may be formed in various configurations (such as corrugations) to facilitate liquid separation from the foods as they cook, for decorative purposes or the like. If desired, the film 14 may be additionally coated onto wall 22 of food receiving cavity 16 (as shown in Fig. 1).

Film 14 will be quite thin, preferably on the order of about 75 to about 400A, most preferably about 100 to about 200A. As will be more fully described ;s, 1

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SS S 55 5 0 hereinafter, films can be chosen with significantly different film thicknesses, yet with similar absorptivu or reflective properties.

The titanium nitride film is preferably deposited onto the selected substrate by a magnetron sputtering process where a titanium target is sputtered in an atmosphere that includes at least some nitrogen gas. Sputtering is a well-known technique for forming a layer of material on a substrate. Reactive sputtering 10 is where one constituent is sputtered in the presence of a gas of another. When the material to be sputtered is an electrical conductor, a dc potential is used. When the material to be sputtered is an insulator, it is preferred to use an rf potential in order to eliminate 15 the build-up of surface charges on the insulator and the resulting loss in accelerating potential. The sputtering rate can be greatly increased by confining the plasma to a region adjacent to the target. Such confinement intensifies the plasma and increases both the probability of gas atom-electron collisions and the probability that the ions thus formed will strike the cathode. By use of a magnetic field, the plasma can be confined to a region which is only slightly separated from the cathode. One such device is the planar magnetron sputtering cathode.

The stoichiometry of titanium nitride films of the invention can vary. Thus, the films may be represented as TiN,, where x is between about 0.8 to about 1.3. Where, for example, the titanium target is :0o sputtered in the presence of a gas that is all nitrogen, then x will be greater than i, usually on the order of about 1.1 to about 1.3. Where the titanium target is sputtered in a gas mixture reduced in the amount of nitrogen (for example, to include an inert gas and nitrogen), then x will normally be less than i. With, 0 0 S 0 6 0 S S

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for example, a gas mixture that is 70 wt.% argon and wt.% nitrogen, then x will be about 0.8.

Films of the invention preferably have a sheet resistivity of between about 40 to about 1000 ohms/sq., more preferably from about 200 to 900 ohms/sq. and most preferably (for a CPET substrate) between about 500-800 ohms/sq. Table I illustrates data taken of titanium nitride films on CPET substrates with varying film thicknesses. The targets used were all titanium.

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Gas Composition Sheet Film Percentage Final During Resistivity Thickness Absorbed Terrp.

Sputtering (ohms/sq.) Power (c 15 100% N, 586 162 15 200 100% N 2 635 104 9 207 70%Ar/30%N, 565 109 16 218 570%Ar/30%N 2 640 93 14 201 The final temperature data of Table I was taken after six minutes exposure of the inventive containers in a 1600W microwave oven set on high at 2450M4Hz. Th e microwave absorption was measured at 2450M4Hz and 0.1mW sees incident power. As can be seen, the data of Table I 25 shows that similar resistivities, absorbed power and final temperatures can be obtained with varying film thicknesses and different gas mixtures during the deposition process to vary stoichiometry.

The relationship between sheet resistivity, absorbed power, transmitted power, and reflected power is illustrated by the data of Table II, which was taken from titanium nitride films on a flat pane of glass.

6 TABLE II Gas Composition Sheet Percentage Percentage Percentage During Resistivity Absorbed Transmitted Reflected Spttering (ohms/sq.) Power Power Power 100% N 2 340 36 34 100% N 2 390 41 36 23 100% N 2 445 42 36 22 100% N 2 570 34 43 23 10 100% N 2 960 21 56 22 0 *e

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Control Substrate (uncoated) 1 79 For the same film stoichiometry, lower sheet resistivities indicate thicker films while higher sheet resistivities indicate thinner films. As can be seen from the data of Table II, one can have comparable absorption with different film thicknesses (as seen by the 340 and 570 ohm/sq. values). While the data of Table II is for films coated onto flat planes of glass, analogous results and relationships are obtained with substrates of the invention.

Titanium nitride films of the invention are durable and provide rapid, uniform heating. Table III illustrates data showing film heating as a function of time from microwave heating. The heat produced was measured using infra-red thermography.

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Temperature (fC) 140 157 158 156 155 156 155 156 152 155 154 155 Time (Seconds) 100 110 120 The data of Table III shows the films can reach about 150-160"C in 20 seconds and substantially uniformly maintain that temperature. The substrates of Table III were CPET trays. For comparison, CPET trays were coated with stainless steel. The stainless steel coated trays reached only about 85 0 C after 2 minutes heating.

Whert containers of the invention define one food receiving cavity with a film of the invention and 25 another cavity that is substantially microwave transparent, then the microwave transparent cavity should be masked during deposition of the film.

Dual cavity containers were prepared where one cavity was coated with titanium nitride and the other cavity was masked by nesting a conforming CPET component in the non-coated cavity. The depositions were uniform using this method of masking and were repeatable.

Containers of the invention were then tested by placing 8 on a sheet of 3mm glass in a microwave oven and run for seconds at high power. The containers were measured on the side next to the non-coated cavity as close to the bottom as possible. The data of Table IV illustrates the tempereA-.res after 45 seconds, the film thicknesses and the sheet resistances. The containers had some evidence of melting on the upper edge, or lip, of the cavities. This slight edge melting can be eliminated by masking the top lip of the containers 10 during deposition.

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TABLE IV Temperature(OC) Thickness Ohm/sq.

93 100 738 127 154 670 143 162 586 a 6

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EXAMPLE I Example I specifically describes the preparation of preferred embodiment containers. The vacuum system used to deposit titanium nitride on CPET trays was an Airco Coating Technology G-series system single-ended with two deposition zones each containing two planar magnetron cathodes.

The single-ended system has a mechanically pumped entry and exit lock which can be evacuated to microns pressure. This is followed by the deposition zones which can be evacuated via diffusion pumps to 1 x 6 torr. For deposition purposes, the deposition zones can be filled to the 1 x 10- 3 torr range with inert or reactive gases. Deposition occurs by conveying the i 1 9 substrate back and forth under the cathode until the desired film thickness is achieved. The coater uses Airco Coating Technology HRC-3000 cathodes capable of uniformly coating a maximum substrate size of 40" x One coat zone was used with two titanium targets. The Ti was sputtered in 100% nitrogen using an MKS gas flow control system to regulate N 2 flow at 888 sccm for A sputtering pressure of 1.5 x 10 3 torr. Two Ti cathodes were run at a constant power of 20 kW each.

*e 10 The conveyor line speed was 200" per minute. The coating was deposited in two passes under the cathode.

Each deposition run was made with five trays on a 2' x 3' glass carrier. A tray was placed in each corner and one in the middle. Two of these trays were used as test samples with a glass microscope slide placed in the tray. The slide was used to measure film thickness and sheet resistance.

The trays had two food receiving cavities. One of the cavities was intended for foods not desired to be 41. 20 crisp cooked (such as various vegetables). The second food receiving cavity was corrugated and was coated with the titanium nitride film on the corrugated bottom and along the surrounding wall. The one cavity intended for vegetables was masked by nesting another CPET vegetable 25 cavity tray into the one cavity.

As may be seen by the data of Table V, containers coated within each run showed excellent sheet resistance uniformity and comparison between runs shows good reproducibility.

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a b c ohm/sacb 650 645 645 645 637 637 *00 0 owe 20 00 00 255 703 695 695 690 682 682 679 673.

671 648 641 641 658 650 650 608 601 601 550 544 544 564 558 558 589 583 583 615 608 608 ease 0

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11 Although the present invention has been described with reference to specific examples, it should be understood that various modifications and variations can be easily made by those skilled in the art without departing from the spirit of the invention.

Accordingly, the foregoing disclosure should be interpreted as illustrative only and not to be interpreted in a limiting sense. The present invention is limited only by the scope of the following claims.

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Claims (9)

1. A container, useful in microwave heating of foods, comprising: a substrate being substantially microwave transparent and having a determinable melting or scorching temperature; and, a titanium nitride film on at least a portion of the substrate, the film being adapted to absorb microwave energy, to become heated and to transfer heat to foods carried by the container.

2. The container as in claim 1 wherein the film becomes heated to a temperature less than the substrate melting or scorching temperature when the container is exposed to microwave energy of up to 16000W at 2450 MHz for up to 6 minutes.

3. The container as in claim 1 or claim 2 wherein the film has a sheet resistivity in the range from 40 to 1000 ohms/square. 16 17 18 19 20 21 22 23 24 25 26 27 28 29 31 32 33

4. wherein The container as in any one of claims 1 to 3 the substrate is crystallized polyethylene terephthalate.

5. The container as in any one of claims 1 to 4 wherein the film consists essentially of TiNt where x is from 0.8 to 1.3.

6. The container as in any one of claims 1 to wherein the film has a thickness in the range from 75 to 400 A.

7. A microwavable container comprising: a tray defining a food receiving cavity, the cavity having a thin film adhered thereto with a sheet resistivity

900110. ldspe.003,39924.spe,2 34 36 37 8 1 0 F.- N I in the range from 200 to 900 ohms/square, the consisting essentially of a titanium nitride. film

8. The container as in claim 7 wherein the food receiving cavity is formed of crystallized polyethylene terephthalate.

9. The container as in claim 7 or claim 8 wherein the tray defines a first and a second food receiving cavity, the thin film being adhered to the first cavity and adapted to transfer heat to foods when placed therein, the second cavity being substantially microwave transparent. A container substantially as hereinbefore described with reference to the drawings and/or the Example. DATED this 10th day of January, 1990. The BOC Group, Inc By its Patent Attorneys DAVIES COLLISON 16 17 18 S 19 20 21 22 23 24 26 27 Sa: 28 2 31 32 33 34 36 37 900110.eldspe.003,39924.spe,3